In the early 1970s, communications carriers began offering communications networks designed for transmission of digital data. In 1974, the Federal Communications Commission approved the Bell System's Dataphone digital service (DDS). DDS is a synchronous facility providing full-duplex service. Primary channel speeds are 2.4, 4.8, 9.6, and 56 kilobits per second.
Terminal access to the DDS network is accomplished by means of a digital service unit, which alters serial unipolar signals into forms of modified bipolar signals for transmission and returns them to serial unipolar signals at the receiving end.
These DDS links may be either point-to-point or multipoint. Point-to-point links establish a direct connection between two points in a network. Multipoint links are analogous to "party lines" used for telephone voice communications. They permit one line to link all terminals to a central location, such as a main computer. The advantage of multipoint service is that the sharing of a line reduces the cost. However, problems arise if two terminals attempt to send data at the same time.
For a multipoint line structure to be effective, one station must be the master, or central processing, station, while the other stations are slaves. A special control unit is used as an interface between the master location and the slave locations. This control unit, known as a multipoint junction unit (MJU), permits the interconnection of a number of terminals (slaves) to a central site (master) via data communications lines. The function performed by the MJU is commonly referred to as "data bridging".
In operation, line discipline imposed by the user of the multipoint network seeks to prevent more than one terminal from transmitting data at any time, although two or more terminals may receive data simultaneously. This line discipline combines polling and selecting techniques and requires that each terminal on the line have a unique address, as well as circuitry to respond to a message sent to that address. Standards for MJUs are published in a document entitled Digital Data System (DDS) Multipoint Junction Unit Requirements, Technical Advisory No. TA-TSY-000192, published by Bell Communications Research, Inc.
A limitation of existing MJU devices is that they are restricted to transmissions at the standard DDS data rates, which provide a maximum data rate of 56 kilobits per second. However, these data rates are slow compared to the faster rates now used in switched telephone systems, such as the 1.544 megabits per second rate of T1 carriers or fractional T1 carriers. There is a growing need to provide digital data bridging at rates greater than 56 kilobits per second. Examples of applications that need this functionality are digital video teleconferencing and data transfers between central and branch offices of institutions such as banks that handle voluminous data.
As an alternative to the standard MJU, many data communications manufacturers have developed proprietary multipoint bridging units. These have the advantage of providing for higher data rates, but require special equipment for ensuring synchronization of the data on each line at the receiving end. Also, being proprietary, these units have no standard method for testing. A need exists for a method of using existing MJUs to provide bridging at higher data rates.